This invention concerns a coherent pulse radar system and process with no distance ambiguity for the detection of a target presenting flashes of very short duration and high period, notably helicopters.
Certain surveillance radars seek in particular to detect agitated targets whose equivalent radar surface may not be great enough to be detected except during very short moments. This may be the case, for example, for helicopters which need to be recognized as such. An essential characteristic of helicopters, from the point of view of the radar cross-section, is that their main rotor blades give a very high cross-section only when they are in the neighborhood of a plane perpendicular to the radar-helicopter direction. This configuration is repeated periodically with a high period, for example, about 20 to 100 milliseconds, but lasts only for a very short time, giving rise, for example, to a "flash" of a duration of approximately 50 to 200 microseconds.
For a pulse-type radar to have a sufficient probability of detecting these blades, an interval between pulses must be chosen which is of the same order of magnitude as the duration of the blade flashes. If a radar with periodic pulses is used, which is necessary in order to eliminate parasite echos properly, periods of repetition must be used. This leads to maximum non-ambiguous distances of the order of twenty kilometers, thus too short in many cases for the distance coverage required.
A first solution envisaged is to use pulses of high repetition frequency and, to eliminate the distance ambiguities, to emit successive bursts of pulses with different repetition periods. A disadvantage of this solution is that the target observation time necessary is greatly increased, so it is nearly impossible to track the target.
Another known solution, described for example in French patent no. 77 38830, consists of using blocks of pulses with a constant repetition frequency which is different from one block to another. These blocks are interlaced and the pulses from a block being recognized by their carrier frequency, which differs from one block to another. But in this case too, to eliminate the distance ambiguity, the observation time is long.
A solution derived from this one would consist of using interlaced bursts with a constant low repetition frequency (therefore without distance ambiguity for the radar cover envisaged), each burst using a different carrier frequency. However, this solution, like the preceding one, requires N emitters and N receivers of different carrier frequencies, which means complex, bulky and costly equipment.